1. Field of the Invention
The present invention relates to a projection lens system of a projector that enlarges and projects an image displayed on a light valve, such as a liquid crystal panel or a DMD, onto a screen.
2. Description of the Related Art
Rear-projection-type devices are known as image display apparatuses with large screen sizes and slim bodies. It is disclosed in Japanese Laid-Open Patent Publication No. 2001-42211 and in Japanese Laid-Open Patent Publication No. H09-218379 that the height and depth of a projector can be reduced by disposing a prism for bending the optical path at an intermediate position behind a closest lens group to the screen in a lens system so that the optical path is bent by 90° or thereabouts.
When a liquid crystal panel is used as the light valve (light modulator), and in particular when an LCOS (Liquid Crystal on Silicon) panel (i.e., a reflective liquid crystal panel) or a DMD (digital mirror device) is used as the light modulator, the lens system that forms the optical system for projecting the image formed on the light modulator onto the screen needs to have a long back focus. As one example of a wide-angle lens that has a long back focus, there is a retrofocus-type lens in which a first lens group with negative refractive power and a second lens group with positive refractive power are disposed in that order from the screen side. In this type of lens system, it is easy to make the incident side telecentric, so that it is possible to prevent color shading due to color separation in the light modulator and angular dependence of the spectroscopic characteristics of the combining means. However, retrofocus-type lens systems are normally highly asymmetric, which makes it difficult to reduce distortion. Correction of distortion can be facilitated by disposing a positive lens on the screen side of the lens system, but to achieve a lens system that has a wide angle relative to the lens diameter, it is preferable to use a meniscus lens, which is convex on the screen side and has negative refractive power, closest to the screen.
By disposing a cemented lens on a light modulator side of a first lens group and a cemented lens on the screen side of a second lens group respectively, the symmetry of the lens arrangement can be improved, and various kinds of aberration, in particular chromatic aberration, can be favorably corrected. However, cemented lenses are combinations of a positive lens and a negative lens, and so tend to be thick, which increases the length of the first lens group positioned on the screen side. Accordingly, even if the optical path is bent by disposing a prism between the first lens group and the second lens group as shown in the Japanese Laid-Open Patent Publication No. 2001-42211, the part of the bent optical path on the screen side, that is, the first lens group, will still be thick. Accordingly, such lens system is not suited to being enclosed inside a compact, slim rear projector.
It is an object of the present invention to provide a lens system that can have a slim, compact overall construction and in which various kinds of aberration can be favorably corrected, making the lens system suited to a rear projector.